The present invention relates to transmission systems in which xe2x80x9clinexe2x80x9d optical fibers guide optical signals that carry information to be transmitted. The present invention relates more particularly to the case when such fibers are of the single-mode type at the wavelengths used.
Regardless of its type, a line fiber has losses. In long-distance transmission, such losses are compensated by amplifiers or repeaters distributed along the length of a transmission line made up of a succession of such fibers. Such a line is then in the form of a succession of segments, each of which is terminated by an amplifier, and the segments are connected together in series. In order to lower the cost of the system, it is desirable to increase the length of each of the segments to as long as possible. Unfortunately, each amplifier has characteristic noise that increases with increasing gain, and, given the in-line losses, this limits said length.
The line fiber also has chromatic dispersion and a the dispersion has a spectrum gradient. The dispersion depends on the carrier wavelength, i.e. on the wavelength that carries the transmitted signal. The dispersion can deform the signal. In addition, the capacity of the system, i.e. the data rate at which information can be transmitted, is typically increased by multiplexing in which a plurality of carrier wavelengths are used to carry the information. In which case, the gradient of the dispersion prevents the same dispersion value being obtained for each of the wavelengths. Chromatic dispersion values that are too high can give rise to excessive deformation of the signals. That is why, in known manner, the deformation is periodically corrected by dispersion compensators whose dispersion and optionally whose dispersion gradient are opposite in sign to those of the line fiber. Unfortunately, in long-distance transmission, the compensators impart losses, and the higher the dispersion per unit length to be compensated, the higher the losses imparted by such compensators. That is why it is generally desirable to limit the dispersion per unit length of the line fibers.
In early optical fiber transmission systems, the line fibers initially laid were chosen to have low dispersion at the carrier wavelength which was then preferred, namely 1,300 nm. However, it later transpired that a range of wavelengths around 1,550 nm was preferable because that made it possible to use amplifiers having erbium-doped fibers. It then became apparent that the high dispersion values that the initially-laid fibers had at those new wavelengths were incompatible with the desired high data rates. One solution that was then considered was to insert a dispersion compensator at the outlet of each of the line segments of the system. It was implemented because it cost less than laying a new fiber suitable for the new range of wavelengths.
Furthermore, in order to increase the transmission quality of known systems by increasing their signal-to-noise ratio, specialists sought to increase the outlet power level of the amplifiers in those systems. Unfortunately, they were limited by non-linear effects developing and degrading transmission quality. Such effects are significant because the values of the optical electric field that accompanies the propagation of the guided waves are too high. For a given power level of the guided waves, the greater the effective mode area offered to the signals by the line fiber used, the lower the value of that field.
That area depends on the carrier wavelength of the signals and it is defined by the following formula:
Seff=2xcfx80(∫"psgr"2(r)xc2x7rxc2x7dr)2/∫"psgr"4(r)xc2x7rxc2x7dr
in which the two integrals are defined from zero to infinity, r is the distance to the axis of the fiber, and "psgr" is the amplitude of the optical electric field. Said effective area may considered as being the area over which the power of the optical signals is distributed in each right cross-section of said fiber.
In order to increase the signal-to-noise ratio, specialists want that area to be as large as possible. However, they know that if they are to increase that area to as large as possible, they will also have to increase the chromatic dispersion of the fiber. It is well known that such dispersion is the sum of two components whose signs are generally opposite, namely a xe2x80x9cmaterial dispersionxe2x80x9d and a xe2x80x9cwaveguide dispersionxe2x80x9d. Since the material component is imposed by the material and tends to be preponderant, the sum of the two components can be reduced only by increasing the absolute value of the waveguide component. Unfortunately, the waveguide component is proportional to the index difference between the core and the cladding of the fiber. In order to reduce the dispersion of the fiber, it is thus necessary to increase the index difference, which tends to reduce the effective mode area.
That is why, when a new optical fiber transmission system is to be installed over a long distance, it is currently considered that the values of the chromatic dispersion of the line fiber used should be limited, which inevitably limits the effective mode area of the line. That area typically lies in the range 70 xcexcm2 to 120 xcexcm2 for fibers whose dispersion is a few ps/(nmxc2x7km).
An object of the present invention is to make it possible, for limited cost, to provide a long-distance transmission system that uses single-mode fibers and that offers good transmission quality, while also enabling information to be carried at a high data rate.
To this end, the present invention provides, in particular, such a system which uses a single-mode fiber having a very large effective mode area accompanied by high chromatic dispersion and/or a steep spectrum gradient of said dispersion, compensation means being associated with said fiber and compensating at least a major portion of said dispersion and/or of said gradient.
In the context of the invention, it has been found that, in order to make a novel transmission system, dispersion compensators of known type make it possible to compensate appropriately the chromatic dispersion of a line fiber that has an effective mode area that is considerably larger than those of known fibers used in such a system. It has also been found that the losses imparted by the compensators are more than compensated by an increase in the power level of the signals that can then be guided by said fiber without increasing to an excessive extent the non-linear effects affecting the signals in the fiber at the outlets of the amplifiers. The invention thus makes it possible to increase the signal-to-noise ratio appearing at the line outlet, and thus the performance of the system. In addition, the margin that it provides for the chromatic dispersion of the line fiber makes it possible to limit the cost of said fiber.